Data transmission via a relay station with ACK/NACK feedback

ABSTRACT

Techniques for supporting communication by a relay station are described. In an aspect, the relay station may support NACK Type 1 when operating in an amplify-and-forward (AF) mode. The relay station may receive a first transmission of a packet from an upstream station, determine PAPR of the first transmission, and send NACK Type 1 to the upstream station if high PAPR is detected. In another aspect, the relay station may support NACK Type 1 and NACK Type 2 when operating in a decode-and-forward (DF) mode. The relay station may perform PAPR decoding for the first transmission, send NACK Type 1 if PAPR decoding fails, perform channel decoding if PAPR decoding passes, and send NACK Type 2 to the upstream station if channel decoding fails. In yet another aspect, the relay station may operate in the AF mode or the DF mode.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present application for patent is a Continuation of patentapplication Ser. No. 12/775,058 filed on May 6, 2010, entitled DATATRANSMISSION VIA A RELAY STATION WITH ACK/NACK FEEDBACK, and assigned tothe assignee hereof and hereby expressly incorporated by referenceherein.

BACKGROUND

Wireless communication systems are widely deployed to provide variouscommunication content such as voice, video, packet data, messaging,broadcast, etc. These wireless systems may be multiple-access systemscapable of supporting multiple users by sharing the available systemresources. Examples of such multiple-access systems include CodeDivision Multiple Access (CDMA) systems, Time Division Multiple Access(TDMA) systems, Frequency Division Multiple Access (TDMA) systems,Orthogonal FDMA (OFDMA) systems, and Single-Carrier FDMA (SC-FDMA)systems.

A wireless communication system may include a number of base stationsthat can support communication for a number of wireless devices. Awireless device may communicate directly with a base station if acommunication channel between the wireless device and the base stationhas acceptable quality. The wireless device may communicate indirectlywith the base station via a relay station if the communication channelbetween the wireless device and the base station has poor quality. Therelay station may facilitate communication between the wireless deviceand the base station by receiving a first signal from an upstreamstation (e.g., the base station), processing the first signal to obtaina second signal, and forwarding the second signal to a downstreamstation (e.g., the wireless device). The relay station may introduceadditional latency/delay and may require additional processing. It maybe desirable to efficiently support communication between the wirelessdevice and the base station via the relay station.

SUMMARY

Techniques for supporting communication by a relay station are describedherein. In an aspect, the relay station may support negativeacknowledgement (NACK) when operating in an amplify-and-forward (AF)mode. In one design, the relay station may receive a first transmissionof a packet from an upstream station when operating in the AF mode. Therelay station may determine a peak-to-average-power ratio (PAPR) of thefirst transmission. The relay station may send a NACK to the upstreamstation if high PAPR is detected. The relay station may forward thefirst transmission to a downstream station if high PAPR is not detectedand may skip forwarding the first transmission if high PAPR is detected.The upstream station may send another transmission of the packet inresponse to receiving the NACK from the relay station.

In another aspect, the relay station may support negativeacknowledgement of a first type (NACK Type 1) and negativeacknowledgement of a second type (NACK Type 2) when operating in adecode-and-forward (DF) mode. NACK Type 1 is NACK that is sent withoutperforming channel decoding. NACK Type 2 is NACK that is sent due tochannel decoding error. In one design, the relay station may receive afirst transmission of a packet from the upstream station and may performPAPR decoding for the first transmission. The relay station may sendNACK Type 1 to the upstream station if the PAPR decoding fails. Therelay station may perform channel decoding for the packet based on thefirst transmission if the PAPR decoding passes. The relay station maysend NACK Type 2 to the upstream station if the packet is decoded inerror and may send the first transmission of the packet to thedownstream station if the packet is decoded correctly.

In yet another aspect, the relay station may be configurable to operatein either the AF mode or the DF mode. The AF mode or the DF mode may beselected based on one or more criteria such as hop distance, receivedsignal quality, bit error rate (BER), packet error rate (PER), etc. Therelay station may amplify and forward signals to support communicationbetween the upstream and downstream stations if the AF mode is selected.The relay station may decode and forward signals to supportcommunication between the upstream and downstream stations if the DFmode is selected. In one design, the relay station may support NACK Type1 in the AF mode and may support NACK Type 1 and NACK Type 2 in the DFmode.

Various aspects and features of the disclosure are described in furtherdetail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows plots of two signals with different PAPR.

FIG. 3 shows a block diagram of a wireless device, a relay station, anda base station.

FIGS. 4A and 4B show two message flows for data transmission via therelay station in the AF mode with NACK Type 1.

FIG. 5 shows a block diagram of an upstream station, the relay station,and a downstream station, which support the message flows in FIGS. 4Aand 4B.

FIG. 6 shows a message flow for data transmission via the relay stationin the DF mode with NACK Type 1 and NACK Type 2.

FIG. 7 shows a block diagram of the upstream station and the relaystation, which support the message flow in FIG. 6.

FIG. 8 shows a process for supporting communication by the relaystation.

FIG. 9 shows a process for transmitting data by the upstream station.

FIG. 10 shows a process for operating the relay station.

FIG. 11 shows a process for supporting communication by the relaystation.

DETAILED DESCRIPTION

The techniques described herein may be used for various wirelesscommunication systems such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA and othersystems. The terms “system” and “network” are often usedinterchangeably. A CDMA system may implement a radio technology such asUniversal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includesWideband CDMA (WCDMA) and other variants of CDMA. cdma2000 coversIS-2000, IS-95 and IS-856 standards. A TDMA system may implement a radiotechnology such as Global System for Mobile Communications (GSM). AnOFDMA system may implement a radio technology such as Evolved UTRA(E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, Flash-OFDM®, etc. UTRA and E-UTRA are part ofUniversal Mobile Telecommunication System (UMTS). 3GPP Long TermEvolution (LTE) and LTE-Advanced (LTE-A) are new releases of UMTS thatuse E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A and GSM are described indocuments from an organization named “3rd Generation PartnershipProject” (3GPP). cdma2000 and UMB are described in documents from anorganization named “3rd Generation Partnership Project 2” (3GPP2). Thetechniques described herein may be used for the systems and radiotechnologies mentioned above as well as other systems and radiotechnologies.

FIG. 1 shows a wireless communication system 100, which may include anumber of base stations, relay stations, and other network entities. Forsimplicity, only one base station 130 and only one relay station 120 areshown in FIG. 1. A base station is a station that communicates withwireless devices and may also be referred to as a Node B, an evolvedNode B (eNB), an access point, etc. A base station may providecommunication coverage for wireless devices within a particulargeographic area. A base station may communicate with a wireless devicevia the downlink and uplink. The downlink (or forward link) refers tothe communication link from the base station to the wireless device, andthe uplink (or reverse link) refers to the communication link from thewireless device to the base station.

A relay station is a station that receives a transmission of data and/orother information from an upstream station (e.g., a base station) andsends a transmission of the data and/or other information to adownstream station (e.g., a wireless device). A relay station may be astation that is dedicated to relaying transmissions for other stations.A relay station may also be a wireless device that can relaytransmissions for other wireless devices. A relay station may also bereferred to as a relay, a relay base station, etc. A relay station maycommunicate with a wireless device via an access link and maycommunicate with a base station via a backhaul link in order to supportcommunication between the wireless device and the base station.

A wireless device 110 may be stationary or mobile and may also bereferred to as a mobile station, a user equipment (UE), a terminal, anaccess terminal, a subscriber unit, a station, etc. Wireless device 110may be a cellular phone, a personal digital assistant (PDA), a wirelessmodem, a handheld device, a laptop computer, a cordless phone, awireless local loop (WLL) station, etc. Wireless device 110 maycommunicate directly with base station 130 (not shown in FIG. 1) orindirectly with base station 130 via relay station 120 (as shown in FIG.1).

FIG. 1 also shows exemplary transmissions between wireless device 110and base station 130 via relay station 120. For data transmission on theuplink (UL), wireless device 110 may transmit data and controlinformation on an access uplink to relay station 120, which may forwardthe data and control information on a backhaul uplink to base station130. Base station 130 may transmit feedback information on a backhauldownlink to relay station 120, which may forward the feedbackinformation on an access downlink to wireless device 110. For datatransmission on the downlink (DL), base station 130 may transmit dataand control information on the backhaul downlink to relay station 120,which may forward the data and control information on the accessdownlink to wireless device 110. Wireless device 110 may transmitfeedback information on the access uplink to relay station 120, whichmay forward the feedback information on the backhaul uplink to basestation 130. The feedback information sent on one link (e.g., thedownlink) may support data transmission on the other link (e.g., theuplink). The feedback information may comprise channel quality indicator(CQI) indicative of the quality of a communication channel, ACK forpackets decoded correctly, NACK for packets decoded in error, and/orother information.

System 100 may support hybrid automatic retransmission (HARQ) for datatransmission on the downlink and/or the uplink in order to improvereliability of data transmission. For HARQ, a transmitter may send atransmission of a data packet to a receiver and may send one or moreadditional transmissions of the packet, if needed, until the packet isdecoded correctly by the receiver, or the maximum number oftransmissions has been sent for the packet, or some other terminationcondition is encountered. Each transmission of the packet may includedifferent redundancy information for the packet and may be referred toas an HARQ transmission. The receiver may decode the packet based on allHARQ transmissions received for the packet, which may improve thelikelihood of correctly decoding the packet.

System 100 may utilize orthogonal frequency division multiplexing (OFDM)and/or single-carrier frequency division multiplexing (SC-FDM). Forexample, system 100 may be an LTE system that supports OFDM on thedownlink and SC-FDM on the uplink. System 100 may also be a WiMAX systemor a Wi-Fi system that supports OFDM on both the downlink and uplink. Inany case, OFDM and SC-FDM partition a frequency range into multiple(N_(FFT)) orthogonal subcarriers, which are also commonly referred to astones, bins, etc. Each subcarrier may be modulated with data. Ingeneral, modulation symbols are sent in the frequency domain with OFDMand in the time domain with SC-FDM.

OFDM and SC-FDM have certain desirable characteristics such as theability to combat multipath effects. However, a major drawback withOFDM, and to a lesser extend with SC-FDM, is a high PAPR of an outputsignal, which means that the ratio of the peak power to the averagepower of the output signal can be high. For OFDM, the N_(FFT) totalsubcarriers may be independently modulated with data, and high PAPR mayresult from possible in-phase addition of all of the subcarriers whenthey are independently modulated with data. In fact, it can be shownthat the peak power may be up to Q times larger than the average powerfor OFDM, where Q is the number of subcarriers used for transmission.

A high PAPR of an output signal normally requires a power amplifier at atransmitter to be operated at an average power level that may be muchlower than the peak power level, i.e., backed off from peak power. Thisis because large peaks in the output signal may cause the poweramplifier to operate in a highly non-linear region or possibly clip,which may then cause intermodulation distortion and other artifacts thatmay degrade performance. By operating the power amplifier at a back-offfrom peak power, the power amplifier can handle large peaks in theoutput signal without generating excessive distortion. However, theback-off represents inefficient operation of the power amplifier duringother times when large peaks are not present in the output signal.

FIG. 2 shows plots of two output signals 210 and 212 with differentPAPR. The horizontal axis denotes time and the vertical axis denotespower. Output signal 210 average power of P_(AVG2) (which is lower thanP_(AVG1)) and a peak power of P_(PEAK2) (which is higher thanP_(PEAK1)).

To limit the amount of intermodulation distortion, a power amplifier maybe operated at an average power level that is backed off from themaximum power level of the power amplifier. The amount to back off isselected such that the power amplifier does not (or minimally) operatein a highly non-linear region or clip. More specifically, the back-offis normally selected such that the distortion generated by the poweramplifier is limited to a particular level. If the maximum power levelof the power amplifier is equal to the peak power of an output signal,then the back-off may be equal to the PAPR of the output signal. Asshown in FIG. 2, a smaller back-off of BO₁ may be used for output signal210 due to a smaller PAPR, and a larger back-off of BO₂ may be used foroutput signal 212 due to a larger PAPR.

System 100 may support data transmission with feedback of ACK and NACKin order to address high PAPR and improve performance. A transmitter maysend a transmission of a data packet with channel encoding and eitherwith or without PAPR encoding. A receiver may receive and process thetransmission of the packet and may send NACK Type 1 if PAPR encoding isperformed by the transmitter and PAPR decoding by the receiver fails.The receiver may also send NACK Type 1 if PAPR encoding is not performedby the transmitter and high PAPR is detected by the receiver. Thereceiver may perform channel decoding if NACK Type 1 is not sent. Thereceiver may send NACK Type 2 if channel decoding is unsuccessful andmay send an ACK if channel decoding is successful. The transmitter maygenerate another transmission of the packet with different PAPR encodingand/or different channel encoding if NACK Type 1 or NACK Type 2 isreceived and may send this transmission of the packet to the receiver.The processing by the transmitter and receiver are described in furtherdetail below.

FIG. 3 shows a block diagram of a design of wireless device 110, relaystation 120, and base station 130. At wireless device 110, a transmitprocessor 310 may receive data to transmit and may process (e.g.,encode, interleave, and symbol map) the data in accordance with amodulation and coding scheme to obtain data symbols. Transmit processor310 may also process control and/or feedback information to obtaincontrol symbols. Transmit processor 310 may then multiplex the datasymbols, the control symbols, and pilot symbols. As used herein, a datasymbol is a symbol for data, a control symbol is a symbol for control orfeedback information, a pilot symbol is a symbol for pilot or referencesignal, and a symbol may be a real or complex value. Pilot is data thatis known a priori by both a transmitter and a receiver. A modulator(MOD) 312 may process the multiplexed symbols (e.g., for OFDM, SC-FDM,etc.) to generate output samples. Modulator 312 may further condition(e.g., convert to analog, amplify, filter, and upconvert) the outputsamples to generate an uplink signal, which may be routed through aduplexer/switch 316 and transmitted to relay station 120 and/or basestation 130.

At relay station 120, the uplink signal from wireless device 110 may berouted through a duplexer/switch 346 and conditioned (e.g., filtered,amplified, downconverted, and digitized) by a demodulator (DEMOD) 348 toobtain input samples. Demodulator 348 may further process the inputsamples (e.g., for OFDM, SC-FDM, etc.) to obtain received symbols. Areceive processor 350 may process (e.g., symbol demap, deinterleave, anddecode) the received symbols to recover data and other informationtransmitted by wireless device 110.

On the backhaul uplink, a transmit processor 340 may process (e.g.,encode interleave, and symbol map) the data and other informationreceived from wireless device 110 to obtain data symbols and controlsymbols. A modulator 342 may process the data symbols, the controlsymbols, and pilot symbols (e.g., for OFDM, SC-FDM, etc.) to obtainoutput samples. Modulator 342 may further condition the output samplesto generate a backhaul uplink signal, which may be routed throughduplexer/switch 346 and transmitted to base station 130.

At base station 130, the backhaul uplink signal from relay station 120may be received and routed through a duplexer/switch 376, conditionedand processed by a demodulator 378, and further processed by a receiveprocessor 380 to recover the data and other information transmitted byrelay station 120.

On the backhaul downlink, at base station 130, data and otherinformation intended for wireless device 110 may be processed by atransmit processor 370 and further processed and conditioned by amodulator 372 to generate a backhaul downlink signal, which may betransmitted to relay station 120. At relay station 120, the backhauldownlink signal may be received and processed by demodulator 348 andfurther processed by receive processor 350 to recover the data and otherinformation transmitted to wireless device 110.

On the access downlink, at relay station 120, data and other informationfor wireless device 110 may be processed by transmit processor 340 andconditioned by modulator 342 to generate an access downlink signal,which may be transmitted to wireless device 110. At wireless device 110,the access downlink signal may be received and processed by ademodulator 318 and further processed by a receive processor 320 torecover the data and other information transmitted to wireless device110.

Controllers/processors 330, 360 and 390 may control the operation atwireless device 110, relay station 120, and base station 130,respectively. Memories 332, 362 and 392 may store data and program codesfor wireless device 110, relay station 120, and base station 130,respectively.

In one design, relay station 120 may support one or more of the relaymodes shown in Table 1. Relay station 120 may also support differentNACK types for different relay modes, e.g., as shown in Table 1 anddescribed below.

TABLE 1 Relay Modes NACK Relay Mode Description Type Amplify-and-Amplify a received signal and transmit the NACK forward (AF) amplifiedsignal, without decoding the re- Type 1 ceived signal. Decode-and-Decode a received signal to recover data NACK forward (DF) sent in thesignal. Process the data to ob- Types 1 & 2 tain a modulated signal andtransmit the modulated signal.

In an aspect, relay station 120 may support NACK Type 1 when operatingin the AF mode. Relay station 120 may receive a transmission of a packetfrom an upstream station and may perform PAPR detection to detect forhigh PAPR. Relay station 120 may send NACK Type 1 if high PAPR isdetected. Relay station 120 may forward the transmission of the packetto a downstream station if high PAPR is not detected.

FIG. 4A shows a design of a message flow 400 for data transmission viarelay station 120 in the AF mode with NACK Type 1. An upstream station118 may send data transmission to a downstream station 122 via relaystation 120. For data transmission on the downlink, upstream station 118may be base station 130, and downstream station 122 may be wirelessdevice 110. For data transmission on the uplink, upstream station 118may be wireless device 110, and downstream station 122 may be basestation 130. For both cases, data transmission may be sent with channelencoding and without PAPR encoding.

Upstream station 118 may send an HARQ transmission for a packet todownstream station 118 (step 1). Relay station 120 may receive the HARQtransmission and may determine the PAPR of the HARQ transmission (step2). Relay station 120 may send NACK Type 1 to upstream station 118 ifhigh PAPR is detected (step 3). In one design, relay station 120 mayskip forwarding the HARQ transmission to downstream station 122 if highPAPR is detected, as shown in FIG. 4A. In another design, relay station120 may forward the HARQ transmission to downstream station 122 even ifhigh PAPR is detected (not shown in FIG. 4A). Upstream station 118 mayreceive NACK Type 1 from relay station 120 and may adjust one or moretransmission parameters to mitigate high PAPR (step 4). For example,upstream station 118 may increase the back-off of its power amplifierand may operate at a lower average transmit power level if high PAPR isdetected in order to prevent clipping and reduce intermodulationdistortion. Upstream station 118 may send another HARQ transmission forthe packet in response to receiving NACK Type 1 from relay station 120(step 5).

FIG. 4B shows a design of a message flow 410 for data transmission viarelay station 120 in the AF mode with NACK Type 1. Upstream station 118may send an HARQ transmission for a packet to downstream station 118(step 1). Relay station 120 may receive the HARQ transmission and maydetermine the PAPR (step 2). Relay station 120 may forward the HARQtransmission to downstream station 122 if high PAPR is not detected(step 3). Downstream station 122 may receive the HARQ transmission fromrelay station 120 and may decode the packet based on the HARQtransmission (step 4). Downstream station 122 may send ACK if the packetis decoded correctly or NACK Type 2 if the packet is decoded in error(step 5). Relay station 120 may receive the ACK or NACK Type 2 fromdownstream station 122 and may forward the ACK or NACK Type 2 toupstream station 118 (step 6). Upstream station 118 may receive the ACKor NACK Type 2, may terminate transmission of the packet if ACK isreceived (not shown in FIG. 4B) and may send another HARQ transmissionfor the packet if NACK Type 2 is received (step 7).

FIG. 5 shows a block diagram of a design of upstream station 118, relaystation 120, and downstream station 122, which may support message flows400 and 410 in FIGS. 4A and 4B, respectively. At upstream station 118, achannel encoder 510 (which may be part of transmit processor 310 or 370in FIG. 3) may receive a packet of data to transmit to downstreamstation 122 and may process the packet to obtain a corresponding codedpacket. For example, channel encoder 510 may generate a cyclicredundancy check (CRC) for the packet, append the CRC to the packet, andencode the packet and the CRC (e.g., with a convolutional code, a Turbocode, a low density parity check (LDPC) code, a block code, and/or someother code) to obtain the coded packet. For HARQ, channel encoder 510may partition the coded packet into multiple blocks of code bits, oneblock for each HARQ transmission. Each block may include different codebits (i.e., different redundancy information) for the packet. Channelencoder 510 may interleave (or reorder) each block of code bits and maymap the interleaved bits to data symbols. Channel encoder 510 may alsoprocess control information to obtain control symbols.

A modulator 512 may process the data symbols, the control symbols, andpilot symbols (e.g., for OFDM, SC-FDM, etc.) to obtain output samples.Modulator 512 may further condition (e.g., convert to analog, filter,amplify, and upconvert) the output samples to generate a downstreamsignal. The downstream signal may be amplified by a power amplifier (PA)514, routed through a duplexer/switch 516 and transmitted to relaystation 120 and/or downstream station 122.

Upstream station 118 may receive an upstream relay signal from relaystation 120, and the received signal may be routed throughduplexer/switch 516 and processed by a demodulator 518 to obtainreceived symbols. A receive feedback processor 520 (which may be part ofreceive processor 320 or 380 in FIG. 3) may process the received symbolsto recover feedback information sent by relay station 120. The feedbackinformation may comprise (i) NACK Type 1 from relay station 120 and/or(ii) ACK or NACK Type 2 from downstream station 122. Upstream station118 may control data transmission based on the feedback information. Forexample, upstream station 118 may send another HARQ transmission for thepacket if NACK Type 1 is received from relay station 120 or NACK Type 2is received from downstream station 122. Upstream station 118 may adjustthe back-off of power amplifier 514 if NACK Type 1 is received fromrelay station 120. Upstream station 118 may also terminate transmissionof the packet if ACK is received from downstream station 122.

Relay station 120 may receive the downstream signal from upstreamstation 118. The received signal may be routed through a duplexer/switch546 and processed by a demodulator 548 to obtain input samples. A PAPRdetector 550 (which may be part of receive processor 350 in FIG. 3) maycompute the PAPR of an HARQ transmission for a packet based on the inputsamples for the HARQ transmission, as follows:

$\begin{matrix}{{{P\; A\; P\; R} = \frac{\begin{matrix}\max \\k\end{matrix}\left\{ x_{k}^{2} \right\}}{P_{avg}}},{and}} & {{Eq}\mspace{14mu}(1)} \\{{P_{avg} = {\frac{1}{K} \cdot {\sum\limits_{k}x_{k}^{2}}}},} & {{Eq}\mspace{14mu}(2)}\end{matrix}$where x_(k) denotes a complex value for the k-th input sample for theHARQ transmission,

P_(avg) is the average power of the HARQ transmission, and

K denotes the number of input samples used to compute PAPR.

PAPR detector 550 may also compute the PAPR of the HARQ transmission inother manners, e.g., as described by Tarokh et al in a paper entitled“On the computation and reduction of the peak-to-average power ratio inmulticarrier communications,” IEEE Transactions on Communications,Volume 48, Issue 1, January 2000, pages 37-44.

In one design, PAPR detector 550 may compare the computed PAPR against aPAPR threshold, which may be set to a suitable value based on variouscriteria. For example, the PAPR threshold may be set based on decodingcapability of a channel decoder 580 in downstream station 122. The PAPRthreshold may also be set based on channel conditions observed bydownstream station 122, the back-off of power amplifier 514 in upstreamstation 118, and/or other criteria. In general, progressively higherPAPR threshold may be used for progressively more powerful decodingcapability, progressively higher received signal quality, and/orprogressively larger back-off. An appropriate PAPR threshold may bedetermined based on computer simulation, empirical measurement, etc. Inany case, PAPR detector 550 may detect high PAPR if the computed PAPRexceeds the PAPR threshold. If high PAPR is not detected, then thedownstream signal may be conditioned (e.g., filtered and amplified) by amodulator 542 to generate a downstream relay signal, which may be routedthrough duplexer/switch 546 and transmitted to downstream station 122.Conversely, if high PAPR is detected, then PAPR detector 550 may provideNACK Type 1. Feedback information comprising NACK Type 1 may beprocessed (e.g., encoded and symbol mapped) by a transmit feedbackprocessor 540 (which may be part of transmit processor 340 in FIG. 3),further processed by modulator 542, routed through duplexer/switch 546,and transmitted to upstream station 118.

Downstream station 122 may receive the downstream relay signal fromrelay station 120. The received signal may be routed through aduplexer/switch 576 and processed by a demodulator 578 to obtain inputsamples. Demodulator 578 may also perform demodulation on the inputsamples (e.g., for OFDM, SC-FDM, etc.) to obtain received symbols. Achannel decoder 580 (which may be part of receive processor 320 or 380in FIG. 3) may then process (e.g., symbol demap, deinterleave, anddecode) the received symbols to obtain a decoded packet. Channel decoder580 may also check the decoded packet based on the CRC for the packet todetermine whether the packet is decoded correctly or in error. Channeldecoder 580 may provide NACK Type 2 if the packet is decoded in errorand may provide ACK if the packet is decoded correctly.

A transmit feedback processor 570 (which may be part of transmitprocessor 310 or 370 in FIG. 3) may receive feedback information, whichmay comprise ACK or NACK Type 2 from channel decoder 580 and CQI from achannel processor (not shown in FIG. 5). The feedback information may beprocessed (e.g., encoded, interleaved, and symbol mapped) by transmitfeedback processor 570, further processed by a modulator 572, routedthrough duplexer/switch 576, and transmitted to relay station 120 and/orupstream station 118.

FIG. 5 shows an exemplary design of upstream station 118, relay station120, and downstream station 122, which support NACK Type 1 feedback forrelay station 120 operating in the AF mode. NACK Type 1 feedback byrelay station 120 may also be supported in other manners. For example,upstream station 118 may compute the PAPR of an HARQ transmission andmay send the PAPR on an inband channel along with the HARQ transmission,or a paging channel, or a dedicated PAPR channel, or some other channel.Relay station 120 may receive the PAPR of the HARQ transmission fromupstream station 118 and may not compute the PAPR. Relay station 120 maycompare the received PAPR against the PAPR threshold, which may be setbased on the decoding capability of downstream station 122, the channelconditions observed by relay station 120 and/or downstream station 122,the back-off used by upstream station 118, and/or other criteria. Relaystation 120 may also detect for high PAPR in other manners.

As shown in FIGS. 4A, 4B and 5, relay station 120 may support NACK Type1 feedback even when upstream station 118 does not implement PAPRencoding for PAPR reduction. PAPR detection by relay station 120 may beless computationally intensive, may be performed by a “front end” ofrelay station 120, and may have shorter delay. Channel decoding bydownstream station 122 may be more computationally intensive, may beperformed by a “back end” of downstream station 122, and may have longerdelay. Channel decoding may be more powerful but may likely fail whenhigh PAPR is detected. Hence, sending NACK Type 1 for high PAPR mayresult in less delay for retransmission and may reduce computation bydownstream station 122. Sending NACK Type 2 for channel decoding failuremay improve decoding performance due to retransmission. The two levelsof NACK feedback by relay station 120 and downstream station 122 maythus reduce delay and improve performance.

In another aspect, relay station 120 may be configurable to operate ineither the AF mode or the DF mode. The AF mode may have less latency,less processing for relay station 120, and possibly better performanceunder some operating scenarios. The DF mode may have more latency, lowererror rate, and possibly better performance under other operatingscenarios. The AF mode or the DF mode may be selected based on variouscriteria to obtain better performance.

In one design, the AF mode or the DF mode may be selected based on hopdistance. The AF mode may be selected when the hop distance is small andmay have better performance in this case. Conversely, the DF mode may beselected when the hop distance is large and may have better performancein this case.

In one design, the hop distance may be set equal to the larger of (i) afirst distance between relay station 120 and downstream station 118 and(ii) a second distance between relay station 120 and upstream station122. The hop distance may be compared against a distance threshold. TheAF mode may be selected if the hop distance is less than the distancethreshold. The DF mode may be selected if the hop distance is greaterthan the distance threshold. Relay station 120, downstream station 118,and/or upstream station 122 may be mobile, and the first distance and/orthe second distance may vary over time. The hop distance may bedetermined periodically, and the AF mode or the DF mode may be selectedwhenever the hop distance is determined.

In another design, the AF mode or the DF mode may be selected based onexpected channel conditions observed by relay station 120 and/ordownstream station 122. The AF mode may have better performance and maybe selected when channel conditions are good. Conversely, the DF modemay have better performance and may be selected when channel conditionsare poor. Channel conditions may be quantified by a signal-to-noiseratio (SNR), BER, PER, or some other link metric. In one design, SNR maybe set equal to the lower of a first SNR observed by relay station 120and a second SNR observed by downstream station 122. The SNR may becompared against an SNR threshold. The AF mode may be selected if theSNR is greater than the SNR threshold. The DF mode may be selected ifthe SNR is less than the SNR threshold. In another design, BER may beset equal to the higher of a first BER observed by relay station 120 anda second BER observed by downstream station 122. The BER may be comparedagainst a BER threshold. The AF mode may be selected if the BER is lessthan the BER threshold. The DF mode may be selected if the BER isgreater than the BER threshold. The SNR or BER may be determinedperiodically, and the AF mode or the DF mode may be selected wheneverthe SNR or BER is determined. The AF mode or the DF mode may also beselected based on PER or some other link metric in similar manner usinga suitable threshold for the metric.

In yet another design, the AF mode or the DF mode may be selected basedon network management. The AF mode may be selected, e.g., when shorterlatency is desired. Conversely, the DF mode may be selected, e.g., whenlower BER is desired. The AF mode or the DF mode may also be selectedbased on other criteria for network management. For example, the AF modemay be selected if there is a good channel, e.g., the BER of data sentvia the channel is low. If the BER exceeds a threshold, then the DF modemay be selected.

In one design, relay station 120 may periodically transmit pilot orreference signal, which may be used to determine one or more linkmetrics. For example, the pilot or reference signal may be used byupstream station 118 or downstream station 122 to determine round tripdelay (RTD), which may be used to compute hop distance between relaystation 120 and the upstream or downstream station. The pilot orreference signal may also be used to determine channel conditions, e.g.,SNR, signal strength, channel impulse response, etc. Relay station 120may also determine one or more link metrics for the link from theupstream or downstream station to relay station 120 based on pilot orreference signal transmitted periodically by the upstream or downstreamstation.

In yet another aspect, relay station 120 may support NACK Type 1 andNACK Type 2 when operating in the DF mode. Upstream station 118 may senda transmission of a packet with channel encoding and with or withoutPAPR encoding. Relay station 120 may receive the transmission of thepacket from upstream station 118. Relay station 120 may perform PAPRdetection to detect for high PAPR or may perform PAPR decoding. Relaystation 120 may send NACK Type 1 if high PAPR is detected or PAPRdecoding fails. Relay station 120 may decode the packet if high PAPR isnot detected or PAPR decoding passes. Relay station 120 may send NACKType 2 if the packet is decoded in error and may send ACK if the packetis decoded correctly. Upstream station 118 may send another transmissionof the packet if NACK Type 1 or NACK Type 2 is received from relaystation 120.

FIG. 6 shows a design of a message flow 600 for data transmission viarelay station 120 in the DF mode with NACK Type 1 and NACK Type 2.Upstream station 118 may generate an HARQ transmission for a packet withchannel encoding and possibly with PAPR encoding. Upstream station 118may send the HARQ transmission to downstream station 118 (step 1). Relaystation 120 may receive the HARQ transmission from upstream station 118and may perform PAPR detection or PAPR decoding (step 2). Relay station120 may send NACK Type 1 to upstream station 118 if high PAPR isdetected or PAPR decoding fails (step 3). Relay station 120 may performchannel decoding if high PAPR is not detected or PAPR decoding passes(step 4). Relay station 120 may send NACK Type 2 to upstream station 118if channel decoding fails (step 5). Relay station 120 may forward theHARQ transmission to downstream station 122 and may send ACK to upstreamstation 118 if channel decoding passes (step 6).

Downstream station 122 may receive the HARQ transmission from relaystation 120 and may perform PAPR detection, PAPR decoding, and/orchannel decoding (step 7). Downstream station 122 may send NACK Type 1if high PAPR is detected or PAPR decoding fails, or NACK Type 2 ifchannel decoding fails, or ACK if channel decoding passes (step 8).Relay station 120 may receive NACK Type 1, or NACK Type 2, or ACK fromdownstream station 122 and may forward the ACK or NACK to upstreamstation 118 (step 9).

Upstream station 118 may send another HARQ transmission for the packetif NACK Type 1 or NACK Type 2 is received from relay station 120 ordownstream station 122 (step 10). Upstream station 118 may terminatetransmission of the packet if ACK is received from relay station 120 ordownstream station 122.

FIG. 7 shows a block diagram of another design of upstream station 118and relay station 120, which may support message flow 600 in FIG. 6. Atupstream station 118, a channel encoder 710 (which may be part oftransmit processor 310 or 370 in FIG. 3) may receive a packet of data tosend to downstream station 122 and may process the packet to obtain acorresponding coded packet. For HARQ, channel encoder 710 may partitionthe coded packet into multiple blocks of code bits, one block for eachHARQ transmission. Channel encoder 710 may also interleave (or reorder)each block of code bits and may map the interleaved bits to datasymbols.

A PAPR encoder 712 (which may also be part of transmit processor 310 or370) may process each block of data symbols for PAPR reduction and mayprovide a corresponding block of output symbols having lower PAPR in anoutput signal. PAPR encoder 712 may implement a PAPR reduction techniquesuch as selective mapping (SM), partial transmit sequences (PTS), etc.For selective mapping, PAPR encoder 712 may map a block of data symbolsto a set of symbol sequences, all representing the same information.PAPR encoder 712 may then select the symbol sequence with the lowestPAPR for transmission. For partial transmit sequences, PAPR encoder 712may rotate the phase of a block of data symbols to obtain a set ofsymbol sequences conveying similar information and may select the symbolsequence with the lowest PAPR for transmission. PAPR encoder 712 mayprovide side information, which may indicate the selected symbolsequence being transmitted. PAPR encoder 712 may also implement otherPAPR reduction techniques. PAPR encoder 712 may also pass controlsymbols and pilot symbols received from channel encoder 710. The symbolsfrom PAPR encoder 712 may be processed by a modulator 714, routedthrough a duplexer/switch 716, and transmitted to relay station 120.

Relay station 120 may receive the signal from upstream station 118. Thereceived signal may be routed through a duplexer/switch 744 andconditioned by a demodulator 746 to obtain input samples. Demodulator746 may further process the input samples (e.g., for OFDM, SC-FDM, etc.)to obtain received symbols. A PAPR decoder 748 (which may be part ofreceive processor 350 in FIG. 3) may perform PAPR decoding on thereceived symbols in a manner complementary to the PAPR encodingperformed by upstream station 118. The PAPR decoding may be performed(i) based on the side information if sent by upstream station 118 or(ii) blindly for all possible symbol sequences that could have beentransmitted. PAPR decoder 748 may provide NACK Type 1 if PAPR decodingfails and may provide demodulated symbols if PAPR decoding passes. Achannel decoder 750 (which may also be part of receive processor 350 inFIG. 3) may process (e.g., symbol demap, deinterleave, and decode) thedemodulated symbols to obtain a decoded packet. Channel decoder 750 mayalso check the decoded packet based on the CRC for that packet todetermine whether the packet is decoded correctly or in error. Channeldecoder 750 may provide NACK Type 2 if the packet is decoded in error orACK if the packet is decoded correctly.

Relay station 120 may generate feedback information, which may compriseNACK Type 1 from PAPR decoder 748 or NACK Type 2 or ACK from channeldecoder 750. The feedback information may be processed by a transmitfeedback processor 740 (which may be part of transmit processor 340 inFIG. 3) and further processed by a modulator 742, routed throughduplexer/switch 744, and transmitted to upstream station 118.

Upstream station 118 may receive the signal from relay station 120, andthe received signal may be routed through duplexer/switch 716 andprocessed by a demodulator 718 to obtain received symbols. A receivefeedback processor 720 (which may be part of receive processor 320 or380 in FIG. 3) may process the received symbols to recover the feedbackinformation sent by relay station 120. Upstream station 118 may controldata transmission based on the feedback information. For example,upstream station 118 may generate another HARQ transmission with (i) newPAPR encoding if NACK Type 1 is received or (ii) new channel encodingand new PAPR encoding if NACK Type 2 is received. Upstream station 118may then send the HARQ transmission for the packet. Upstream station 118may also terminate transmission of the packet if ACK is received fromrelay station 120.

FIG. 7 shows an exemplary design of upstream station 118 and relaystation 120, which support two-level ACK/NACK feedback. For the firstlevel, PAPR decoder 748 may perform PAPR decoding on the receivedsymbols and determine whether a received symbol sequence is sufficientlyclose (e.g., in distance) to the selected symbol sequence (if sideinformation is sent) or sufficiently close to a valid symbol sequence(if side information is not sent). PAPR decoder 748 may provide NACKType 1 if the received symbol sequence is not sufficiently close to theselected symbol sequence or the valid symbol sequence. Relay station 120may send NACK Type 1 to upstream station 118, which may transmit anothersymbol sequence to relay station 120 in response to NACK Type 1. Relaystation 120 may skip channel decoding if NACK Type 1 is generated andsent.

For the second level, channel decoder 750 may decode the demodulatedsymbols from PAPR decoder 748 to obtain a decoded packet and may performCRC check on the decoded packet. Channel decoder 750 may provide NACKType 2 if the packet is decoded in error and may provide ACK if thepacket is decoded correctly. Relay station 120 may send NACK Type 2 orACK to upstream station 118, which may send another HARQ transmission inresponse to NACK Type 2 or may terminate transmission of the packet inresponse to ACK.

In another design, upstream station 118 may generate an HARQtransmission for a packet with channel encoding and without PAPRencoding. In this design, upstream station 118 may be implemented asshown in FIG. 5. Relay station 120 may perform PAPR detection andchannel decoding and may be implemented with PAPR detector 550 in FIG. 5and channel decoder 750 in FIG. 7.

As noted above, relay station 120 may be configurable to operate ineither the AF mode or the DF mode (e.g., based on hop distance or someother criteria). In one design, relay station 120 may support feedbackof NACK Type 1 when operating in the AF mode, e.g., as shown in FIGS.4A, 4B and 5. Relay station 120 may support feedback of NACK Type 1 andNACK Type 2 when operating in the DF mode, e.g., as shown in FIGS. 6 and7.

In yet another aspect, upstream station 118 and downstream station 122may each be configurable and may support single-level or multi-levelNACK feedback. For example, single-level NACK feedback with only NACKType 2 or two-level NACK feedback with both NACK Type 1 and NACK Type 2may be selected for upstream station 118 based on the capability ofrelay station 120, the capability of downstream station 122, channelconditions, etc. Similarly, single-level NACK feedback or two-level NACKfeedback may be selected for downstream station 122 based on thecapability of relay station 120, the capability of upstream station 118,channel conditions, etc.

FIG. 8 shows a design of a process 800 for supporting communication.Process 800 may be performed by a relay station (as described below) orby some other entity. The relay station may receive a first transmissionof a packet from an upstream station (block 812). The relay station maydetermine a PAPR of the first transmission (block 814). In one design,the relay station may compute the PAPR of the first transmission. Inanother design, the relay station may receive the PAPR of the firsttransmission from the upstream station.

The relay station may detect for high PAPR of the first transmission(block 816). In one design, the relay station may compare the PAPR ofthe first transmission against a threshold. The threshold may bedetermined based on decoding capability of the downstream station,channel conditions observed by the downstream station and/or the relaystation, back-off of a power amplifier at the upstream station, and/orother criteria. The relay station may detect high PAPR if the PAPR ofthe first transmission exceeds the threshold.

The relay station may send a NACK to the upstream station if high PAPRis detected for the first transmission (block 818). The relay stationmay forward the first transmission to a downstream station if high PAPRis not detected for the first transmission (block 820). In one design ofblock 820, the relay station may amplify a signal comprising the firsttransmission from the upstream station and may transmit the amplifiedsignal to the downstream station, without decoding the firsttransmission to recover the packet.

The relay station may skip forwarding the first transmission to thedownstream station if high PAPR is detected for the first transmission(block 822). The relay station may receive a second transmission of thepacket if NACK is sent to the upstream station for the firsttransmission. The first and second transmissions may comprise differentredundancy information for the packet and may have different PAPRs.

FIG. 9 shows a design of a process 900 for transmitting data. Process900 may be performed by an upstream station, which may be a base stationfor data transmission on the downlink or a wireless device for datatransmission on the uplink. The upstream station may send a firsttransmission of a packet to a relay station for forwarding to adownstream station (block 912). The upstream station may send a secondtransmission of the packet to the relay station if NACK Type 1 isreceived from the relay station (block 914). NACK Type 1 may be sent bythe relay station due to high PAPR detected for the first transmission.The upstream station may adjust at least one transmission parameter ifNACK Type 1 is received from the relay station. For example, theupstream station may increase the back-off of a power amplifier at theupstream station in response to receiving NACK Type 1. The upstreamstation may also send the second transmission of the packet to the relaystation if NACK Type 2 is received from the downstream station (block916). NACK Type 2 may be sent by the downstream station due to thepacket being decoded in error.

FIG. 10 shows a design of a process 1000 for supporting communication.Process 1000 may be performed by a relay station (as described below) orby some other entity. The relay station may obtain an indication tooperate in an AF mode or a DF mode (block 1012). In one design, therelay station may select the AF mode or the DF mode. In another design,the relay station may receive the indication to operate in the AF modeor the DF mode from a base station or some other network entity. Forboth designs, the AF mode or the DF mode may be selected based on one ormore criteria such as hop distance, received signal quality, BER, PER,etc. For example, the AF mode may be selected if the hop distance isless than a threshold, and the DF mode may be selected if the hopdistance is greater than the threshold.

The relay station may amplify and forward signals to supportcommunication between an upstream station and a downstream station ifthe AF mode is selected (block 1014). In one design, the relay stationmay receive a first signal from the upstream station, amplify the firstsignal to obtain a second signal, and transmit the second signal to thedownstream station.

The relay station may decode and forward signals to supportcommunication between the upstream station and the downstream station ifthe DF mode is selected (block 1016). In one design, the relay stationmay receive a first signal from the upstream station, decode the firstsignal to obtain decoded data, generate a second signal based on thedecoded data, and transmit the second signal to the downstream station.

In one design, the relay station may support PAPR detection in the AFmode. In this design, the relay station may determine the PAPR of asignal received from the upstream station and may send NACK Type 1 tothe upstream station if high PAPR is detected for the signal. In anotherdesign, the relay station may not support PAPR detection in the AF mode.In this design, the relay station may simply amplify and forward thesignal received from the upstream station, without sending NACK Type 1to the upstream station.

In one design, the relay station may support PAPR decoding and channeldecoding in the DF mode. In this design, the relay station may performPAPR decoding for a signal received from the upstream station and mayperform channel decoding for the signal to obtain decoded data if thePAPR decoding passes. The relay station may send NACK Type 1 to theupstream station if the PAPR decoding fails. The relay station may sendNACK Type 2 to the upstream station if the channel decoding fails. Inanother design, the relay station may support PAPR detection and channeldecoding in the DF mode. In this design, the relay station may determinePAPR of the signal received from the upstream station and may send NACKType 1 to the upstream station if high PAPR is detected for the signal.The relay station may perform channel decoding to obtain decoded data ifhigh PAPR is not detected. The relay station may send NACK Type 2 to theupstream station if the channel decoding fails. In yet another design,the relay station may support only channel decoding in the DF mode. Inthis design, the relay station may perform channel decoding to obtaindecoded data and may send NACK Type 2 to the upstream station if thechannel decoding fails.

FIG. 11 shows a design of a process 1100 for supporting communication.Process 1100 may be performed by a relay station (as described below) orby some other entity. The relay station may receive a first transmissionof a packet from an upstream station (block 1112). The relay station mayperform PAPR decoding for the first transmission, e.g., based onselective mapping, or partial transmit sequences, or some other PAPRreduction technique used by the upstream station (block 1114). The relaystation may send NACK Type 1 to the upstream station if the PAPRdecoding fails (block 1116). The relay station may skip channel decodingfor the packet if the PAPR decoding fails.

The relay station may perform channel decoding for the packet based onthe first transmission if the PAPR decoding passes (block 1118). Therelay station may send NACK Type 2 to the upstream station if the packetis decoded in error (block 1120). The relay station may send the firsttransmission of the packet to a downstream station if the packet isdecoded correctly (block 1122).

The relay station may receive a second transmission of the packet fromthe upstream station if NACK Type 1 or NACK Type 2 is sent for the firsttransmission. The relay station may perform PAPR decoding for the secondtransmission and may send NACK Type 1 to the upstream station if thePAPR decoding fails. The relay station may perform channel decoding forthe packet based on the first and second transmissions if the PAPRdecoding passes. The relay station may send NACK Type 2 to the upstreamstation if the packet is decoded in error based on the first and secondtransmissions of the packet, which may be two HARQ transmissions for thepacket.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the disclosure herein may be implemented as electronichardware, computer software, or combinations of both. To clearlyillustrate this interchangeability of hardware and software, variousillustrative components, blocks, modules, circuits, and steps have beendescribed above generally in terms of their functionality. Whether suchfunctionality is implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem. Skilled artisans may implement the described functionality invarying ways for each particular application, but such implementationdecisions should not be interpreted as causing a departure from thescope of the present disclosure.

The various illustrative logical blocks, modules, and circuits describedin connection with the disclosure herein may be implemented or performedwith a general-purpose processor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Ageneral-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thedisclosure herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC. The ASIC may reside in a user terminal. In the alternative, theprocessor and the storage medium may reside as discrete components in auser terminal.

In one or more exemplary designs, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples and designs described herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A method for wireless communication, comprising:obtaining an indication to operate in an amplify-and-forward (AF) modeor a decode-and-forward (DF) mode by a relay station; amplifying andforwarding signals by the relay station to support communication betweenan upstream station and a downstream station if the AF mode is selected;decoding and forwarding signals by the relay station to supportcommunication between the upstream station and the downstream station ifthe DF mode is selected; receiving a signal from the upstream station bythe relay station in the DF mode; sending a negative acknowledgementhaving a first type (NACK Type 1) to the upstream station if highpeak-to-average-power ratio (PAPR) is detected or PAPR decoding failsfor the signal; and sending a negative acknowledgement having a secondtype (NACK Type 2) to the upstream station if channel decoding fails forthe signal.
 2. The method of claim 1, wherein the amplifying andforwarding signals comprises: receiving a first signal from the upstreamstation by the relay station in the AF mode; amplifying the first signalto obtain a second signal; and transmitting the second signal to thedownstream station.
 3. The method of claim 1, wherein the receivedsignal comprises a first signal, and wherein the decoding and forwardingsignals comprises: decoding the first signal to obtain decoded data ifthe high PAPR is not detected for the signal and the channel decodingpasses; generating a second signal based on the decoded data andtransmitting the second signal to the downstream station.
 4. The methodof claim 1, further comprising: receiving a signal from the upstreamstation by the relay station in the AF mode; and sending a negativeacknowledgement having the first type (NACK Type 1) to the upstreamstation if high peak-to-average-power ratio (PAPR) is detected for thesignal.
 5. The method of claim 1, further comprising: performing thepeak-to-average-power ratio (PAPR) decoding for the signal; andperforming the channel decoding for the signal to obtain decoded data ifthe PAPR decoding passes.
 6. The method of claim 1, further comprising:determining the peak-to-average-power ratio (PAPR) of the signal; andperforming the channel decoding for the signal to obtain decoded data ifthe high PAPR is not detected for the signal.
 7. The method of claim 1,wherein the AF mode or the DF mode is selected based on hop distance, orreceived signal quality, or bit error rate (BER), or packet error rate(PER), or a combination thereof.
 8. The method of claim 1, wherein theAF mode is selected if hop distance is less than a threshold, andwherein the DF mode is selected if the hop distance is greater than thethreshold.
 9. The method of claim 1, wherein the obtaining theindication comprises receiving the indication to operate in the AF modeor the DF mode from a base station, the base station being the upstreamstation or the downstream station.
 10. An apparatus for wirelesscommunication, comprising: means for obtaining an indication to operatein an amplify-and-forward (AF) mode or a decode-and-forward (DF) mode bya relay station; means for amplifying and forwarding signals by therelay station to support communication between an upstream station and adownstream station if the AF mode is selected; means for decoding andforwarding signals by the relay station to support communication betweenthe upstream station and the downstream station if the DF mode isselected; means for receiving a signal from the upstream station by therelay station in the DF mode: means for sending a negativeacknowledgement having a first type (NACK Type 1) to the upstreamstation if high peak-to-average-power ratio (PAPR) is detected or PAPRdecoding fails for the signal; and means for sending a negativeacknowledgement having a second type (NACK Type 2) to the upstreamstation if channel decoding fails for the signal.
 11. The apparatus ofclaim 10, further comprising: means for receiving a signal from theupstream station by the relay station in the AF mode; and means forsending a negative acknowledgement having the first type (NACK Type 1)to the upstream station if the high peak-to-average-power ratio (PAPR)is detected for the signal.
 12. The apparatus of claim 10, furthercomprising: means for performing peak-to-average-power ratio (PAPR)decoding for the signal from the upstream station; and means forperforming the channel decoding for the signal to obtain decoded data ifthe PAPR decoding passes.
 13. The apparatus of claim 10, furthercomprising: means for determining the peak-to-average-power ratio (PAPR)of the signal; and means for performing the channel decoding for thesignal to obtain decoded data if high PAPR is not detected for thesignal.
 14. An apparatus for wireless communication, comprising: atleast one processor configured to obtain an indication to operate in anamplify-and-forward (AF) mode or a decode-and-forward (DF) mode, toamplify and forward signals to support communication between an upstreamstation and a downstream station if the AF mode is selected, and todecode and forward signals to support communication between the upstreamstation and the downstream station if the DF mode is selected; areceiver configured to receive a signal from the upstream station in theDF mode; and a transmitter configured to: send a negativeacknowledgement having a first type (NACK Type 1) to the upstreamstation if high peak-to-average-power ratio (PAPR) is detected or PAPRdecoding fails for the signal; and send a negative acknowledgementhaving a second type (NACK Type 2) to the upstream station if channeldecoding fails for the signal.
 15. A non-transitory computer-readablemedium, comprising code for causing at least one computer to: obtain anindication to operate in an amplify-and-forward (AF) mode or adecode-and-forward (DF) mode; amplify and forward signals to supportcommunication between an upstream station and a downstream station ifthe AF mode is selected; decode and forward signals to supportcommunication between the upstream station and the downstream station ifthe DF mode is selected; receive a signal from the upstream station inthe DF mode; send a negative acknowledgement having a first type (NACKType 1) to the upstream station if high peak-to-average-power ratio(PAPR) is detected or PAPR decoding fails for the signal; and send anegative acknowledgement having a second type (NACK Type 2) to theupstream station if channel decoding fails for the signal.